The present invention relates to a squeal preventing disc rotor for effectively preventing a squeal from occurring when a disc brake is operated.
In a disc brake in which a disc rotor rotating with an axle is pressingly sandwiched between friction pads operated by oil pressure or the like to perform braking, it is known that an extremely uncomfortable high frequency sound called a brake squeal is generated depending on the hardness of the disc rotor, the affinity between the rotor and the friction pads, or the like, when the disc rotor is pressingly sandwiched by the friction pads.
As a countermeasure to the generation of such a high frequency sound, a metal annular member is fitted in a disc rotor at its outer circumference as disclosed in Japanese Patent Unexamined Publication No. Sho-56-164236, or a hole or a groove is formed in a rotor at its braking surface so as to shift a resonant point as disclosed in Japanese Utility Model Unexamined Publication No. Sho. 54-108880.
Recently, however, various kinds of materials have been used as friction materials. For example, when a material in a group of semi-metallic or non-asbestos is used as friction pads, it is impossible to sufficiently suppress the squeal by means of the countermeasure for suppressing vibrations as described above. In order to investigate the cause, a brake test was actually performed by use of friction of a group of semi-metallic or non-asbestos materials. As a result, it was found that a squeal is caused mainly due to a so-called longitudinal wave (a compressional wave) in which the front and rear surfaces of a rotor vibrate in the directions opposite to each other. FIG. 7 shows the direction of wave transmission in the rotor disc at arrow 70. Arrows 72 and 74 show the directions of vibration, and nodal lines of vibration 76 are formed.
The fact was proved by the detection that the resonance point of a longitudinal wave obtained in an excitation test for measuring a vibration characteristic of a single rotor product as shown in FIG. 8 coincides with the frequency of a squeal in an actual car test using pads of a group of non-asbestos as shown in FIG. 9. This result applied to an actual car test using pads of a group of semi-metallic material.
FIG. 8 shows a vibration (longitudinal wave) application test result. Arrow 80 in FIG. 8 shows the direction of the longitudinal wave transmission. In the graph of FIG. 8, the overplotted dark areas at points 82, 84 and 86 are the Second mode resonance, the Fourth mode resonance, and the Sixth mode resonance, respectively, for the resonance point corresponding to squeal and resonance point of longitudinal wave. FIG. 9 shows an actual car squeal test result, showing background noise 90, noise (squeal) corresponding to Second resonance mode of longitudinal wave 92, Fourth mode of resonance 94, and the Sixth mode of resonance 96.
As a result of a further experiment in which friction pads of a group of non-asbestos were pressingly fitted on each of disc rotors which were different in diameter from each other and which were used in four actual cars (cars A, B, C and D) respectively to thereby investigate the frequency characteristics, the data shown in FIG. 10 was obtained. Being apparent from FIG. 10, portions at which noises are caused by longitudinal waves concentrate on resonance modes formed in even number orders, that is, the second, fourth, and sixth orders. This is because the frequencies in the other even number order modes exceed the human audible range of frequency to thereby cause no problem. The reason why noise is not generated in any odd mode is that the odd number mode has no resonance point because a disc or ring-like body such as a rotor has no open end unlike a rod-like body having open ends so that compressional waves circulating in the rotor solid body interfere with each other to cancel the odd modes. FIG. 10 shows the calculated frequency of the longitudinal waves of the sixth mode 106, the Fourth mode 104 and the Second mode 102 as a function of the rotor outer diameter. FIG. 10 also shows a depiction of the Second mode longitudinal waves 112 and the Fourth mode longitudinal waves 114, as they travel around the disc rotor.